The invention is in the field of nuclear magnetic resonance (NMR) and pertains especially to superconducting probe coils realized on planar substrates.
In an NMR apparatus of any type for the study of the properties of a body, the coupling of the apparatus to the body is critical and this coupling occurs through the agency of the probe coil. The probe coil applies the RF magnetic field to the body for excitation of resonance and, typically, also serves as the component on which the resonant signal is impressed by inductive coupling to the body or sample. The sensitivity of the apparatus depends to a great extent upon the nature and efficiency of this coupling. Recent years have seen the introduction of superconducting probe coils using high temperature superconducting (HTSC) films deposited on planar substrates in various geometries.
Factors which contribute to an efficient NMR coil include filling factor "xgr", quality factor (Q) and intrinsic coupling efficiency xcex6. The filling factor is a geometric feature which measures the portion of the interior volume defined by the coil which is available to sample. Clearly, a large quantity of sample, which is excited and resonantly de-excites, contributes a larger signal. The coil may be observed to be characterized by a total inductance, however any portion of the capacitance which does not contribute to detected signal represents a loss of sensitivity. Q represents the ratio of the power stored in the field of the resonant circuit to the resistive losses in the resonant circuit. Sensitivity is proportional to Qxc2xd. Accordingly, it is desirable to maximize the quantities "xgr", xcex6 and Q.
HTS coils exhibit extraordinary high values of Q, ranging into the order of 104. These coils are fabricated on planar substrates due to properties of the HTSC materials. Coils and coil pairs for this application are treated in the prior art. See, for example, U.S. Pat. No. 5,565,778. Unfortunately, the critical current, e.g., the maximum current which can be sustained in these HTSC conductors is limited due to perturbations of the superconducting phase.
In prior art, simple planar HTSC coils were known for NMR applications. See, for example, U.S. Pat. No. 6,201,392. It is known to employ a plurality of HTSC conductors in an arrangement of nested loops. See U.S. Pat. No. 5,594,342 to Brey and Withers. Each loop, as such, contributes an inductive reactance and to the extent that the nested loop is adjacent portions of inner and outer neighboring loops, a distributed capacitive reactive component is realized.
A requirement of the invention is to obtain a very high degree of uniformity in the RF magnetic field imposed on a selected volume. There is also an independent desire to avoid non-uniformity in the current density supported by the conductor(s) forming the coils. When such current density non-uniformity is experienced in HTSC devices, the superconducting phase may become unstable.
The geometry of the preferred embodiment of the invention is a nested arrangement of M (M, an integer) elemental spiral portions. The M elemental spirals exhibit a progression of dimensions to permit the nested arrangement. Each elemental spiral has an inner and an outer end. The inner (outer) ends of adjacent nested spiral elements are geometrically displaced by an angular amount xcex94xcex8 to obtain a RF offset of xcex94"PHgr" in relative phase. Preferably, the same relative phase is maintained for all adjacent pairs. Each adjacent pair of elemental spirals contribute a distributed capacitance coupling to the parallel inductances of the coil whereby a desired RF resonance characteristic is achieved.
It is desired to increase the RF current carrying capability of an HTSC probe coil and to achieve uniformity in the current density distribution over the coil. This is accomplished in a resonant structure comprising a plurality of closely adjacent concentric spiral conductors. Each spiral conductor supports a standing wave and the several spiral conductors are disposed to provide a phase shift relative to the adjacent spiral conductor(s). The magnitude of distributed capacitance between adjacent capacitors is thus a matter for design choice.
Multi-resonant HTS coils are realized with planar concentric phase shifted (rectangular) spirals with the inner coil designed for the high frequency resonance. The two coils are in close proximity on one planar dimension and relatively distal relationship on the other dimension, in order to direct the flux return regions for the two coils respectively to the outer periphery of the assembly for the outer coil and to the inter coil region for the inner coil. Opposite helicity for these coils is preferable where greater isolation is desired.
The coupling of the above multi-resonant coil to an RF source or sink may be selectively enhanced by the alignment of a coupling loop in respect to the flux return of the respective nested coils. Coupling to the outside coil portion may simply involve placing the projection of a simple coupling loop over the area defined by the outside loop. Selective coupling to the inner coil is accomplished with a butterfly loop, e.g. a series pair of loops of opposite helicity, the projection of which straddles the inside and outside portions of the inner loop.